During operation of electrodynamic machines, for example large generators or motors, electrical voltages occur between the shaft and the housing or ground. The shaft can be grounded at one end of the machine. The shaft is therefore isolated from ground at the other end of the machine, because of the voltages which occur. Insulated bearings, for example, are used for this purpose.
If this isolation is electrically conductively bridged because of component faults, and so forth, electric currents flow in the shaft which can damage the shaft and adjacent components such as the bearings.
There is therefore a desire to detect the occurrence of such insulation faults and currents in order then to protect the machine against more extensive damage, by suitable measures.
It is known from the prior art (see for example the document U.S. Pat. No. 6,300,701) for the shaft current to be measured by a Rogowski coil which is arranged around the shaft, and for the shaft current to be monitored for a predetermined maximum value. An appropriate warning alarm is triggered if this predetermined value is exceeded.
Similar solutions based on induction coils are used in commercially available appliances (for example the “RARIC” system from the ABB Company or the “DRS Compact2A” system from the VA Tech SAT Company).
In the abovementioned methods and apparatuses, the induction coils or Rogowski coils do not operate ideally and can also produce signals which do not originate from the current through the shaft but are induced by other fields. The stray field from the main magnetic field of the machine can produce an interference signal such as this.
In order to avoid measurement errors resulting from stray fields, the abovementioned “RARIC” appliance can be set such that suitable filtering ensures that only the third mains harmonic of the recorded signal is evaluated. The mains harmonics are in this case the signal components whose frequencies are an integer multiple of the fundamental mains frequency. This is based on the assumption that the interference fields dominantly have only the fundamental harmonic. The magnitude of these third harmonics is subjected to limit-value assessment.
However, in practice, it has been found that, despite filtering, the measurement is still highly susceptible to errors. The limit values are therefore set relatively high, in order to avoid false alarms. In order to avoid bearing damage, it should, however, be possible to reliably detect even small currents in the shaft.